The present invention relates generally to acousto-optic devices, and more particularly to absorbing surface acoustic waves in acousto-optic devices.
Integrated acousto-optic devices function through the interaction of surface acoustic waves (SAWs) on a crystal surface with light traversing the crystal. An example of an acousto-optic device is shown in FIG. 1. The device is a 2.times.2 tunable optic switch having optical waveguides 10, 11, 17, 18, 19, and 20, with two bending sections 22a and 22b along a region of optical waveguides 17 and 18. The device also has acoustic waveguides 12 and 13, piezoelectric transducers 14 and 15, and polarization splitters 21. The device is formed on the surface of a crystal 16, such as LiNbO.sub.3. When electrically stimulated, transducers 14 and 15 produce SAWs that travel through acoustic waveguides 12 and 13 and interact with light passing through optical waveguides 17 and 18.
The inventor has recognized, however, that non-ideal behavior of the SAW gives rise to parasitic effects. First, the SAW may be scattered due to inhomogeneities on the surface of the crystal. Inhomogeneities can be caused, for example, by indiffusion of dopants, surface impurities, additional surface layers, or by the borders of the substrate. The scattered SAW disturbs the acousto-optic interaction between the SAW and the light traveling through the waveguides.
The SAW can be scattered, or it can be reflected by the inhomogeneities, or by the acoustic waveguides. Scattered or reflected acoustic waves can interfere with other acoustic waves. The resulting interference pattern changes with the wavelength of the SAW resulting in wavelength dependent characteristics of the acousto-optic device.
Further, the structure that generates the SAW (e.g., piezoelectric transducers 14 and 15 of FIG. 1) does not necessarily generate directed waves. Rather, the waves irradiate in various directions, degrading device performance.
Other problems associated with conventional acousto-optic devices include internal reflections of the transducer that result in fluctuations of transducer efficiency with frequency. Also, waves propagating inside the substrate ("bulk-waves") can interfere with those propagating on the surface.
Finally, In acousto-optic devices like the one shown in FIG. 1, a coupling between acoustic waveguides 12 and 13 may develop. If such coupling develops, the efficiency of both converters Is decreased. To function effectively, then, the waveguides 12 and 13 must be separated by a relatively large distance (typically 200 microns). This required separation limits the degree to which the devices can be integrated.
U.S. Pat. No. 6,002,349 discusses the integration of acousto-optic filters and switches. The objects of the invention are to provide an efficient acousto-optic structure on an x-cut piezoelectric substrate and to provide an acousto-optic filter and coupler that do not incur an optical frequency shift and that are polarization independent To achieve these objects, the invention includes a two-stage acousto-optic filter fabricated as an integrated circuit with an acoustic absorber between the two stages.
European Patent Application EP 0737880A1 discusses an acousto-optical waveguide device for wavelength selection. As shown in FIG. 4. the device includes an acoustic absorbing means placed along an acoustic waveguide 41, close to the end of optical waveguide 36 that is connected to coupler 37 to absorb the residual acoustic wave. It also includes an acoustic absorbing means placed close to the end of waveguide 34 that is connected to coupler 32 to absorb the acoustic wave generated by transducer 44 propagating in a direction opposite to the optical signals.
UK Patent Application (Publication No. GB 2,304,917) entitled "Integrated Optical Devices" discloses a tunable filter having an acoustic waveguide parallel to an optical waveguide. The application discloses two methods of controlling the power density of an advancing acoustic wave and, thus, the power transfer to the optical wave. First, the acoustic waveguide can have a varying cross-sectional area. Second, the energy of the acoustic wave can be absorbed along the length of the acoustic waveguide.
Finally, a publication entitled "Passband Engineering of Acousto-Optic Tunable Filters" by R. S. Chakravarthy et al. (1995 ECIO Proceedings, paper TuPo, Poster Session, p. 137-40) investigates solutions to the problems of interchannel crosstalk and wavelength misalignment crosstalk in acousto-optic tunable filters. The authors propose, among other things, placing an attenuating overlay symmetrically along the length of the device taking care to avoid the optical waveguides. The device structure is shown in FIGS. 4(a) and 5(a).
The inventor has discovered that these techniques do not solve the problems recognized by the inventor. The inventor has found a need, therefore, for an acousto-optic device that reduces the parasitic effects of surface acoustic waves.